The present invention relates to signal processing techniques for providing high fidelity signal amplification with high power efficiency. More specifically, the present invention provides techniques by which distortion in all classes of amplifiers including, for example, power supply transitioning artifact in a class G amplifier, may be reduced to provide a low distortion output signal.
In general, improvements in the power efficiency of electronic amplifiers and line drivers are desirable. In communications, a line driver is used to drive telecom or datacom signals on physical interfaces such as, for example, subscriber loop copper lines. Likewise, in audio/industrial applications, electronic amplifiers are used to drive speakers or other electrical loads. Power transfer efficiency is a critical design issue especially for applications characterized by a high peak-to-average power ratio such as, for example, the driving of copper subscriber loops or high fidelity audio loudspeakers. In such applications, it is not unusual for output power levels to range from a few hundred milliwatts to tens or even hundreds of watts. It is also typical for such applications that the peak output power far exceeds the average power delivered to the load, i.e., that the periods of high power dissipation constitute a relatively small percentage of the operating time of the driver or amplifier. And, even though it far exceeds the output voltage swing for the great majority of circuit operation, the supply voltage must be large enough to exceed the output voltage swing during periods of peak power dissipation in order to keep high signal fidelity (or low distortion). Thus, during periods in which the output voltage swing is relatively low, i.e., most of the time, the efficiency of the circuit is very low due to the fact that the supply voltage is much greater than it needs to be.
Power efficiency is further degraded due to the fact that supply voltages tend to be set significantly higher than the predicted peak output voltage swing to maintain output signal distortion within acceptable limits. That is, as the output voltage of an amplifier approaches the power supply rails, signal distortion tends to increase dramatically. As a result, most circuit designers tend to use supply voltages which exceed the expected peak output voltage swing by as much as 20% or more.
An example of the inefficiency of an application in which the peak-to-average power ratio is 5 will be instructive. If the average power transmitted is 200 mW into a 50 ohm load, the RMS value of the output voltage is 3.162 volts and the average peak value is 1.4142*3.162=4.47 volts. That is, most of the time, the output signal swings between .+-.4.47 volts. For a peak-to-average ratio of 5, this means that the output voltage can swing as high as 5*3.162=15.81 volts. To maintain appropriately low distortion levels during periods of peak power dissipation, such a peak output voltage justifies the use of a .+-.15 volt supply. This results in a dissipation of greater than 800 mW to deliver 200 mW into the load. Thus, in such a configuration, even a well designed class AB amplifier will operate at less than 20% efficiency.
Class D amplifiers, i.e., switching amplifiers, address the problem of power efficiency but, in general, are characterized by more signal distortion than comparable analog amplifiers, especially as the output signal swing approaches the power supply rails. In addition, such amplifiers are particularly susceptible to degradation from electromagnetic interference (EMI). A switching amplifier has been developed by Tripath Technology, Inc. of Santa Clara, Calif., which improves upon switching amplifier technology generating output signals having distortion levels comparable with high fidelity analog amplifiers. An example of such a switching amplifier is described in U.S. Pat. No. 5,777,512 for METHOD AND APPARATUS FOR OVERSAMPLED, NOISE-SHAPING, MIXED-SIGNAL PROCESSING issued Jul. 7, 1998, the entirety of which is incorporated herein by reference for all purposes.
However, at switching frequencies significantly above audio bandwidths, the oversampling employed by the improved switching amplifier of the above-referenced patent may result in switching losses which begin to undermine power efficiency gains. That is, for communications applications such as ISDN and the various xDSL standards where bandwidths range from 30 kHz to 1.2 MHz, the switching losses in the output devices in switching applications may be unacceptably high; particularly for a switching technique which relies on oversampling and thus has an even higher switching rate than standard switching techniques.
One approach to improving power efficiency for high peak-to-average ratio applications has been the so called Class G amplifier for which the output signal remains analog in nature. SGS Thompson has used this class of amplifiers for audio applications, an example of which is shown in FIG. 1. During operation, amplifier 102 is alternately connected to two different supply voltages .+-.5V and .+-.15 V. When the output peak signal is less than roughly 4.3V (depending upon the voltage of zener diode 104), then the .+-.5V supply is connected to amplifier 102 via diode 112. When the output peak signal exceeds 5 volts less the voltage of diode 104, then the .+-.15V supply is connected to amplifier 102 via NPN transistor 108. That is, when the output signal is below a certain level, amplifier 102 is connected to the .+-.5V supply via diodes 112 and 114. When the output voltage rises high enough, amplifier 102 is connected to the .+-.15V supply.
Through the power supply switching technique described with reference to FIG. 1, a class G amplifier can improve power efficiency by varying degrees depending upon how often the signal is diverting power to 5V. A class G amplifier always has a higher average efficiency than a class AB amplifier if the signal being passed is not a DC signal greater than some threshold. Unfortunately, while this improvement can be significant, there is a corresponding increase in output signal distortion due to the power supply transition noise coupling into the circuit. This coupling can also be mixed to other frequencies by the nonlinearities of the amplifier/output stage. In communications applications, increased signal distortion leads to an increased bit error rate. In audio applications, increased distortion leads to a perceptible degradation in sound quality. Thus, despite its power efficiency improvements, the standard class G amplifier is unacceptable for applications in which signal distortion must be kept low.
Another tradeoff in linear amplifier design is bias current vs. linearity. Generally speaking, a higher bias current results in a more linear output. By contrast, reducing bias current, while improving power efficiency, results in increased distortion as the load gets smaller. This is a classical and well documented tradeoff in the design of a variety of amplifier classes, e.g., class A, AB, etc.
It is therefore desirable to provide an amplifier topology with an increased power efficiency which generates an output signal with low enough distortion, for example, for use in communications, high fidelity audio applications, and in any application with a high peak-to-average ratio.